This invention relates to a method and apparatus for removing water from a mixture containing both water and ethanol.
At present water is usually removed from such a mixture by distillation but ethanol forms a binary azeotrope with water and consequently, by using simple distillation techniques, it is impossible to remove all of the water. When producing a potable spirit azeotrope strength spirit is the maximum ethanol concentration that is produced. Water is removed from such a binary azeotrope to produce a substantially water-free ethanol for industrial use by adding a third component to produce a ternary system. Upon subsequent distillation of the ternary system substantially water-free ethanol is produced. To obtain substantially anhydrous ethanol using this technique requires a substantial quantity of energy and, in the past, a number of proposals have been made to remove the water more efficiently. For example, it is known to treat an azeotrope mixture with a desiccant such as fused sodium or potassium acetate and thereby remove water from such a mixture. It is also known to use a molecular sieve dryer operating in either the liquid or the vapour phase to remove the last remaining traces of water from substantially water-free ethanol but, in general, the use of a molecular sieve has been confined to such a polishing role.
In addition to these conventional techniques specific proposals are described in U.S. Pat. Nos. 3,132,079, 4,273,621 and EP-A-No. 0158754 involving the use of a molecular sieve to remove water from an azeotrope vapour resulting from distillation and containing an organic liquid and water. In the first of these a method is discussed in which a molecular sieve is used to adsorb water from a vapour phase azeotrope mixture of water and isopropanol. To regenerate the molecular sieve some of the water-free isopropanol so produced is heated and passed through the molecular sieve in the reverse direction to remove water from the water saturated molecular sieve. This wet isopropanol is then returned to the distillation system. This conserves the isopropanol but the system is inefficient and uses a great deal of energy firstly in the distillation of the mixture to obtain the vapour phase azeotrope and secondly to heat the dry isopropanol so produced and use this to regenerate the molecular sieve. This results in wetting the isopropanol again and hence reducing the yield of dry isopropanol whilst, at the same time, requiring the use of still further energy to redistil the re-wetted isopropanol.
In the second of these proposals, a system for removing water from a vapour phase ethanol/water azeotrope is described in which an ethanol/water mixture is subjected to high pressure distillation at a pressure of 7.5 bar. The resulting vapour phase azeotrope is then diluted with a carrier gas consisting of carbon dioxide or nitrogen and passed through a molecular sieve to remove the water vapour. The ethanol is allowed to condense out and the carrier gas is used in the regeneration of the water saturated molecular sieve. By careful choice of operating temperatures and pressures and also using the heat of adsorption and desorption it is possible to use very little energy for the removal of the water from the vapour phase azeotrope but, of course, the high pressure distillation part of the process does require a considerable amount of energy. Moreover, this system is entirely concerned with the removal of water from the vapour and not only relies on it being a vapour but on it being a vapour resulting from a high pressure distillation system. This system could not be applied to a liquid feed stock unless that liquid feed stock was vaporized initially and this would also require considerable quantities of energy.
In the third of these a process is described in which a carbon molecular sieve is used to remove water from an ethanol/water azeotrope vapour and it is postulated that such molecular sieves could also be used to remove water from ethanol in the liquid phase.
Other proposals have been made, for example in EP-A-No. 0122539 and U.S. Pat. No. 4,420,561 to adsorb ethanol onto a molecular sieve material to remove it from an ethanol/water mixture and then recover the ethanol upon regenerating the molecular sieve material.